MFC 297809,297810,297811

MFC 297809,297810,297811

297809
    hyperv/hn: Cap default # of rings to 8.

    8 gives the best performance in both Azure and local Hyper-V on both
    10Ge and 40Ge. More rings are still allowed by manual configuration.

    Reviewed by:        Dexuan Cui <decui microsoft com>
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5879

297810
    hyperv/hn: By default enable multiple TX/RX rings, aka vRSS.

    Reviewed by:        Dexuan Cui <decui microsoft com>
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5880

297811
    hyperv/hn: Remove unnecessary NULL checks

    Submitted by:       Jun Su <junsu microsoft com>
    Reviewed by:        sephe
    MFC after:  1 week
    Sponsored by:       Microsoft OSTC
    Differential Revision:      https://reviews.freebsd.org/D5905

git-svn-id: svn://svn.freebsd.org/base/stable/10@301952 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f